This invention relates in general to fluorinated dimethyl ethers and specifically to methyl difluoromethyl ether as a starting material for the synthesis of fluorinated dimethyl ethers. Such fluorinated dimethyl ethers, including bis(difluoromethyl)ether (CHF.sub.2 OCHF.sub.2), have utility as CFC alternatives, particularly for use as refrigerants, blowing agents, etc.
Bis(difluoromethyl)ether has been prepared previously by chlorination of dimethyl ether followed by isolation and fluorination of bis(dichloromethyl)ether. The chlorination step resulted in a complex mixture of chlorinated dimethyl ethers, some of which were unstable, e.g. to distillation, from which bis(dichloromethyl)ether was separated. Moreover, chloromethyl methyl ether and bis(chloromethyl)ether are produced by this reaction, and are carcinogens.
The higher chlorinated methylethers also have been made by using either chloromethyl methylether or bis(chloromethyl)ether as starting materials. The chlorination of both these ethers, in either vapor phase or solution, is not nearly as vigorous as that of dimethyl ether, permitting easier control of the reaction. However, this approach also involves the use of the same carcinogenic ethers.
Another approach to the synthesis of methyl difluoromethyl ether is disclosed by Hine and Porter in Methylene derivatives as intermediates in polar reaction VIII. Difluoromethyl in the Reaction of Chlorodifluoromethane with Sodium Methoxide, published in the Journal of the American Chemical Society 79, 5493-6 (1957). This article describes a reaction mechanism wherein the desired difluoromethyl-methyl-ether is synthesized in a batch reaction in a fixed ratio with the by-product trimethyl-orthoformate, while continuously refluxing the unreacted feed. However, not only does this reaction produce large amounts of trimethylorthoformate, but also the product itself breaks down to trimethylorthoformate, resulting in less than advantageous yields of the desired difluoromethyl methyl ether.
U.S. Pat. No. 5,185,474, the disclosure of which is hereby incorporated by reference, discloses avoiding the production of such carcinogens and unstable compounds by using methyl difluoromethyl ether as a starting material. The methyl difluoromethyl ether is chlorinated to produce a reaction mixture including at least one compound of the formula CF.sub.2 HOCH.sub.3-z Cl.sub.z, wherein z is 1, 2, or 3. The mixture can then be fluorinated, or any one of the chlorination compounds first separated from the mixture and separately fluorinated.
However, the chlorination of an alkyl group has a high heat of reaction, which is absorbed by the reactant and products as a function of their specific heats, and then transferred through the walls of the reactor which serve as heat transfer agents. Unstable reactants and/or products, such as CF.sub.2 HOCH.sub.3 and its chlorinated derivatives, have limited thermal stability and can decompose to some extent in such a system because of the temperature gradients and hot spots present in the reaction mixture.
Accordingly, it is an object of the present invention to provide an improved process for the chlorination of difluoromethylmethylether by removing the heat of reaction.